This invention relates to medical ventilators having a free standing bellows and, more particularly, to a system for determining a bellows collapsed condition.
In general, medical ventilator systems are used in the administration of anesthesia to a patient undergoing an operation and to maintain the patient under anesthesia until the cessation of the operation. Such systems include ventilators which provide a breath to the patient and which typically include a bellows in the system to separate the breathing circuit to which the patient is connected from the drive gas emanating from the ventilator. This is normally done in order to allow the partial reuse of the breathing circuit gases on successive breaths from the patient.
An advantage of such rebreathing in anesthesia systems is that the rebreathing of the gases allows the reuse of the expensive anesthetic agents that are added to such breathing gases. Thus, utilization of the anesthetic agent is reduced and the cost of using such agent is minimized.
Medical ventilator systems can incorporate several different types of bellows arrangements, including hanging, driven and standing type bellows. An example of a system incorporating a standing type bellows is shown and described in U.S. Pat. No. 5,315,989 of Tobia and which is assigned to the present assignee and the disclosure of which is hereby incorporated by reference, in that system, the standing bellows is driven pneumatically in a downward direction (gravity added) by the ventilator to expel the gas from inside the bellows to the patient circuit for breathing by the patient. The bellows is then allowed to rise back to its original position when the ventilator drive pressure is released as the patient exhales and additional fresh gas is admitted to the system, returning the bellows to its full up position.
By design, a standing bellows will collapse if its entire gas volume is exhausted. This is a relatively common occurrence that can be caused by uncompensated leaks or high ventilation demand. Under such conditions, it is possible for the bellows to deflate entirely and be pressed against the bottom of the bellows container by the drive pressure that continues to be administered to the interior of the bellows container.
When the bellows collapsed condition occurs, it is valuable to be able to alert the clinician of that condition so that remedial action can be taken. It is also desirable to prevent the ventilator from continuing to pin the bellows at the base of the bellows container with ever increasing drive pressure in a fruitless effort to deliver additional volume to the patient. Such conditions can seriously overpressure and stress the drive circuit of the ventilator. In addition, with the advent of closed loop control controlled ventilators, it is important to detect and react to this condition in order to prevent volume over-delivery on breaths subsequent to the causative condition being corrected and the bellows volume refilled.